<style>
  .foo::part(base) {
    background-color: rgba(255, 0, 0, 0.8);
    padding: 10px 0;
    color: #fff;
    font-family: "Gill Sans", sans-serif;
  }
</style>

<b-marquee
  style="--banana-marquee-color: orange; --banana-marquee-font-size: 12px; --banana-marquee-line-height: 2; --banana-marquee-background-color: rgba(0,0,0,.5);"
  content="Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua."></b-marquee>
<b-marquee class="foo"
  content="Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua."></b-marquee>